Let us begin with the following Andreev equation for unconventional superconductors:
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\(\left(\begin{matrix}- {\rm i}{\mathbf {v_{\rm F}}}\cdot \nabla & \Delta ({\mathbf {k}_{\rm F}},{\mathbf {r}}) \cr \Delta ({\mathbf {k}_{\rm F}},{\mathbf {r}}) & {\rm i}{\mathbf {v_{\rm F}}}\cdot \nabla \end{matrix}\right)\left(\begin{matrix}u_E({\mathbf {r}}) \cr v_E({\mathbf {r}})\end{matrix}\right)= E\left(\begin{matrix}u_E({\mathbf {r}}) \cr v_E({\mathbf {r}})\end{matrix}\right).\) 
